This invention relates to a method for recovering hydrocarbons from a subterranean hydrocarbon-bearing formation containing low gravity viscous oils or bitumens. More particularly, this invention relates to recovery of hydrocarbons from tar sands by thermal means.
The recovery of viscous oils from formations and bituments from tar sands by conventional methods has generally been unsuccessful because of the high viscosity and low mobility of the oil or bitumens. While some success has been realized in stimulating recovery of heavy oils by the use of thermal methods, essentially no success has been realized in recovering bitumens from tar sands. Bitumens can be regarded as highly viscous oils having a gravity in the range of about 5 to 10.degree. API and contained in an essentially unconsolidated sand. These formations containing bitumens are referred to as tar sands. One such deposit is Athabasca tar sands located in Alberta, Canada, which deposit is estimated to contain several hundred billion barrels of oil.
Among the conventional thermal recovery methods applied to produce viscous hydrocarbons from formations and bitumens from the tar sands are steam injection, hot water injection and in-situ combustion. In employing steam and hot water injection the viscous hydrocarbons are heated to a temperature at which their viscosity is sufficiently reduced and their mobility is sufficiently improved so as to enhance their flow through the pores of the formation. In the use of in-situ combustion, much higher temperatures are realized whereby the in-place crude or bitumen also undergoes at least partial thermocracking or visbreaking to increase its mobility.
In operating the conventional in-situ combustion process an oxygen-containing gas such as air is introduced into the formation via a well and combustion of the in-place crude adjacent the wellbore is initiated by one of many accepted means, such as the use of a downhole gas-fired heater or downhole electric heater or chemical means. Thereafter, the injection of the oxygen-containing gas is continued so as to maintain the high temperature combustion front which is formed, and to drive the front through the formation toward the production well.
As the combustion front advances through the formation, a swept area consisting, ideally, of a clean sand matrix, is created behind the front. Ahead of the advancing front various contiguous zones are built up that also are displaced ahead of the combustion front. These zones may be envisioned as a distillation and cracking zone, a condensation and vaporization zone, an oil bank and a virgin, or unaltered zone.
The temperature of the combustion front is generally in the range of 750.degree.-1200.degree. F. The heat generated in this zone is transferred to the distillation and cracking zone ahead of the combustion front where the crude undergoes distillation and cracking. In this zone, a sharp thermal gradient exists wherein the temperature drops from the temperature of the combustion front to about 300.degree.-450.degree. F. As the front progresses and the temperature in the formation rises, the heavier molecular weight hydrocarbons of the oil become carbonized. These coke-like materials are deposited on the matrix and are the potential fuel to sustain the progressive in-situ combustion.
Ahead of the distillation and cracking zone is a condensation and vaporization zone. This zone is a thermal plateau and its temperature is in the range of from about 200.degree. to about 450.degree. F., depending upon the distillation characteristics of the fluids therein. These fluids consist of water and steam and hydrocarbon components of the crude.
Ahead of the condensation and vaporization zone is an oil bank which forms as the in-situ combustion progresses and the formation crude is displaced toward the production well. This zone is high oil saturation contains not only reservoir fluids but also condensate, cracked hydrocarbons and gaseous productions of combustion which eventually reach the production well from which they are produced.
Various improvements relating to in-situ combustion are described in the prior art that relate to the injection of water, either simultaneously or intermittently with the oxygen-containing gas to scavenge the residual heat in the formation behind the combustion front, thereby increasing recovery of oil. Prior art also discloses regulating the amount of water injected so as to improve conformance or sweep and to control the combustion.
Despite the use of these thermal recovery techniques none has been particularly successful since long periods of time and considerable amounts of thermal energy are required to heat up a formation sufficiently to obtain the desired reduction in viscosity and improved mobility.
There is the additional difficulty caused by the fact that heavy oils have a high percent of high molecular weight components. These high molecular weight components are carbonized ahead of the advancing combustion front and form the potential fuel for sustaining in-situ combustion. Because of the high percent of these high molecular weight components, there is an undesirable high fuel requirement and consequent low recovery. A further difficulty that occurs is that because of the low rate of propagation of in-situ combustion, the displaced hydrocarbons are cooled and hence become more immobile, thereby causing blockage of the formation, with the result that the progress of the front is impeded and the combustion cannot be sustained.
The difficulties recited above become compounded when these techniques are applied to the tar sands, because not only do the tar sands have a low gravity, i.e., 6.degree.-8.degree. API and a higher viscosity, i.e., in the millions of centipoises; but also the permeability is so low that difficulty has been experienced in establishing fluid communication within the formation.
The present invention seeks to overcome these difficulties by the application of a combination of thermal techniques that can best exploit the advantages of each technique.